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(libc.info)openlog

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openlog
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   The symbols referred to in this section are declared in the file
`syslog.h'.

 - Function: void openlog (char *IDENT, int OPTION,
     int FACILITY)

     `openlog' opens or reopens a connection to Syslog in preparation
     for submitting messages.

     IDENT is an arbitrary identification string which future `syslog'
     invocations will prefix to each message.  This is intended to
     identify the source of the message, and people conventionally set
     it to the name of the program that will submit the messages.

     `openlog' may or may not open the `/dev/log' socket, depending on
     OPTION.  If it does, it tries to open it and connect it as a
     stream socket.  If that doesn't work, it tries to open it and
     connect it as a datagram socket.  The socket has the "Close on
     Exec" attribute, so the kernel will close it if the process
     performs an exec.

     You don't have to use `openlog'.  If you call `syslog' without
     having called `openlog', `syslog' just opens the connection
     implicitly and uses defaults for the information in IDENT and
     OPTIONS.

     OPTIONS is a bit string, with the bits as defined by the following
     single bit masks:

    `LOG_PERROR'
          If on, `openlog' sets up the connection so that any `syslog'
          on this connection writes its message to the calling process'
          Standard Error stream in addition to submitting it to Syslog.
          If off, `syslog' does not write the message to Standard
          Error.

    `LOG_CONS'
          If on, `openlog' sets up the connection so that a `syslog' on
          this connection that fails to submit a message to Syslog
          writes the message instead to system console.  If off,
          `syslog' does not write to the system console (but of course
          Syslog may write messages it receives to the console).

    `LOG_PID'
          When on, `openlog' sets up the connection so that a `syslog'
          on this connection inserts the calling process' Process ID
          (PID) into the message.  When off, `openlog' does not insert
          the PID.

    `LOG_NDELAY'
          When on, `openlog' opens and connects the `/dev/log' socket.
          When off, a future `syslog' call must open and connect the
          socket.

          *Portability note:*  In early systems, the sense of this bit
          was exactly the opposite.

    `LOG_ODELAY'
          This bit does nothing.  It exists for backward compatibility.

     If any other bit in OPTIONS is on, the result is undefined.

     FACILITY is the default facility code for this connection.  A
     `syslog' on this connection that specifies default facility causes
     this facility to be associated with the message.  See `syslog' for
     possible values.  A value of zero means the default default, which
     is `LOG_USER'.

     If a Syslog connection is already open when you call `openlog',
     `openlog' "reopens" the connection.  Reopening is like opening
     except that if you specify zero for the default facility code, the
     default facility code simply remains unchanged and if you specify
     LOG_NDELAY and the socket is already open and connected, `openlog'
     just leaves it that way.